The invention relates to a heat-resistant nonwoven fabric made of bonded synthetic staple and/or endless fibers and to a method of manufacturing same. The nonwoven fabric is characterized by an especially good continuous heat resistance such as is desired in numerous applications, as for example in electrical insulation.
It is known to use nonwoven fabrics in the electrical insulation art, for example as supporting materials for insulating resins. Nonwovens consisting of 100% polyester fibers are commonly used for this purpose, being made into pre-impregnated products known as "pre-pregs" for later fabrication. That is, they are impregnated with a reactive insulating resin, usually on the basis of epoxides and polyimides, and the resin is transformed by a heat treatment to the "B-stage" in which the resin is not yet fully hardened. The final hardening is performed after the fabricated part has been installed, in a transformer or an electric motor, for example. In some cases, however, the nonwoven fabric is fabricated without preimpregnation, and then one of the above-mentioned resins is applied to it to produce the insulating properties. Such nonwoven fabrics combined with insulating resins can be used with great reliability as insulation material as long as the temperatures to which they are continuously exposed in operation do not exceed 165.degree. C.
Increasingly stringent requirements in the electrical insulating art necessitate a search for insulating materials of greater stability, especially materials of improved resistance of continuous exposure to heat. There is an increasing demand for insulating materials which can resist continuous exposure to temperatures up to 250.degree. C. Indeed, the short-term ability of such materials to withstand heat must be even higher, and should be of the order of about 350.degree. C. For example, extremely fine glass fabrics and papers on the basis of polyaryl amides can be used within the above-stated temperature range. Such products, however, usually do not provide the desired uniformity of structure, and in addition they are often too coarse, since not only the ability to withstand heat is required, but also great fineness of the fibers--less than 200.mu., and preferably from about 30 to 80.mu.. To assure optimum insulation even at high voltages, the insulating resin must be absorbed very uniformly by the nonwoven fabric serving as the support; this is not the case with the materials known hitherto. The invention is therefore addressed to the problem of developing a heat-resistant and at the same time fine, bonded nonwoven fabric of very uniform porosity.